Origin of the Invention
This invention was made in the course of or under contract no. DE-AC03765F00098 between the U.S. Department of Energy and the University of California. The government has certain rights in this invention.
Field of the Invention:
The present invention concerns a process for altering the chemical composition of aerogels without affecting their morphological properties and under conditions which inhibit formation of a plasma within the aerogels' pores. Specifically, the aerogels are exposed to gas and irradiated at optimal temperatures and pressures.
Description of Related Art:
Present processes to produce inorganic aerogels are limited to the synthesis of oxides of metals and metalloids or pure carbon aerogels. Currently, there are no synthetic routes to non-oxide inorganic aerogels, such as metal carbides, nitrides, silicides, and halides.
General methods of preparing aerogels have been known as early as the 1930's. See, for example S. S. Kistler, in Nature 127:741 (1931) and S.S. Kistler in U.S. Pat. No. 2,249,767 both of which describe the preparation of silicon dioxide aerogels. The methods in Kistler involve techniques which were considered laborious and dangerous at that time, including drying gels formed from sodium silicate and removing solvents by supercritical extraction. Because of the hazards involved with Kistler's process, there was very little progress in aerogel research for several decades thereafter.
In the early 1980's there was a resurgence of aerogel research. Present developments in aerogels have lead to a variety of compositions. However, despite significant advances in aerogel technology, most of the aerogels produced are limited to the oxides of metal or metalloids. See, for example T. M. Tillotson, et al. U.S. Pat. No. 5,409,683. Organic aerogels are also known. See, for example, R. W. Pekala, et al. J.Non-Cryst. Solids 145:90 (1922). Methods exist to convert some of these organic aerogels into pure carbon aerogels by pyrolysis in an inert atmosphere. Other processes to prepare aerogels include R. S. Upadhye, et al., U.S. Pat. No. 5,227,239; T. M. Tillotson, et al., U.S. Pat. No. 5,275,796; and C. Colmenares, U.S. Pat. No. 5,030,607. Yet, there is still a need for procedures to produce non-oxide inorganic aerogels and in particular, procedures for making non-oxide aerogel from other types of aerogels.
General procedures for the transformation of metals or metalloid oxides into non-oxide materials have not been successful with aerogels. Such techniques require treatment with a reducing agent at temperatures which are too high to preserve an aerogel's structure. For example, the conversion of SiO.sub.2 to Si.sub.3 N.sub.4 using ammonia occurs at 1450.degree. C. See, T. Ishii, et al. Jap. Pat. #63,162,514 (Jul. 6, 1988). However, silica aerogels will shrink and densify at above 450.degree. C. Therefore, typical procedures to convert metal or metalloid oxides into non-oxide organic aerogels are not useful.
Prior attempts at modifying aerogels are also not applicable to making non-oxide inorganic aerogels. These modification techniques have been restricted to the addition of a single layer or deposits of new material within the aerogel. Such procedures do not significantly change the chemical formula of the aerogels.
Some art of general interest is as follows:
A. J. Hunt, et al. J Non-Cryst. Solids Vol. 185, p.227 (1995) disclose chemical vapor infiltration to deposit new material within the aerogel matrix.
H. Yokogawa, et al.J. Non-Cryst. Solids Vol. 186, p.23 (1995) disclose a method of affecting one molecular layer of an aerogel, leaving the bulk of the material unchanged.
P. H. Tewari, et al. in U.S. Pat. No. 4,610,863 disclose a process for forming a transparent aerogel as an insulating array.
J. Carlson, et al. in U.S. Pat. No. 4,629,652 disclose a method of production of an aerogel on a support.
S. Reed, et al. in U.S. Pat. No. 5,306,445 (1994) disclose chemical treatment to change one molecular layer of the interior surface of the aerogel.
S. L. Koontz in U.S. Pat. No. 5,314,857 disclose the use of oxygen plasma to remove adsorbed organic material from the interior surface of aerogel powders resulting in the formation of aerogel granules yet not changing the chemical structure of the aerogel.
S. P. Hotaling in U.S. Pat. No. 5,358,776 disclose a plasma processing technique to deposit plasma on a substrate, adding a new material to the substrate but not altering its chemical structure.
W. Cao, et al. in U.S. patent application, Ser. No. 08/221,643 disclose thermally assisted chemical vapor infiltration to deposit a material on the pores of the aerogel.
J. Maire in GB Patent 2 141 418 A disclose chemical deposition in the vapor phase of carbon in silica aerogels.
None of these references individually or collectively teach or suggest the present invention.
All patents, patent applications, articles, references, publications, standards and the like cited in this application are incorporated herein by reference in their entirety.
There is, therefore, a strong need for alternative methods of modifying aerogels to the presently used chemical treatments in order to change the chemical formula of aerogels and produce non-oxide inorganic aerogels. What is needed is a process which alters the functional properties of aerogels and preserves the original aerogel's morphological properties. Further, the improved methods should circumvent the need for excessively high temperatures during the transformation process.
The present invention has operational and commercial value in that it provides efficient processes for modifying the chemical structure of aerogels thereby changing the aerogel's physical properties yet maintaining the original structural properties. Of special interest is the use of the present process in producing non-oxide inorganic aerogels. Non-oxide inorganic aerogels find application in many fields including catalysts, ceramics, composite materials, chemical sensors (e.g. oxygen, etc.), optical devices, light-emitting devices, microelectronics, energy storage devices, etc.